The determination of a functional parameter of an organ, in particular the quantitative determination of liver function, is of great importance in many areas of medicine. Chronic liver diseases are widespread in Europe, with 8.9 million people affected by hepatitis C. As their disease progresses, these individuals or patients find themselves in most cases under permanent medical care. In the therapy and management of patients with chronic liver diseases, quantifying the liver function can greatly improve the therapy control, and assessment of liver function is crucial in ensuring that the correct therapeutic decisions are made.
Partial liver resection is a common method used in surgery today. It is performed as a segmental resection or hemihepatectomy along the anatomical margins. Extensive interventions in the parenchymatous organ were made possible by the development of a wide variety of operating techniques. The post-operative morbidity and mortality after liver failure, however, is still a considerable problem, due to inadequate liver function capacity resulting from previously damaged liver tissue or from there being too little liver tissue remaining. Many of the surgical procedures, however, have to be performed in previously damaged liver tissue, in most cases where the liver has been transformed by cirrhosis. It is therefore necessary to be able to determine the functional liver capacity of a patient before the partial liver resection, so as to ensure that patients who no longer have sufficient functional reserves of liver tissue are not subjected to what is for them a high-risk operation or are not assigned to other treatment methods.
Assessment of liver function is of particular importance in liver transplantation, since here the organ function has to be assessed without delay and a treatment decision has to be made quickly. In many clinical situations, it is also difficult to assess whether there is a parenchymatous disturbance or whether other causes are responsible for the clinical symptoms presented by the patients. In summary, therefore, there is a great need for a genuinely quantitative liver function test for broad application in medicine.
Efforts are therefore being made across the world to develop simple tests that allow prognostic statements to be made concerning the functional reserves of liver cell tissue. Conventional laboratory parameters are very unreliable and therefore unsuitable for this purpose. They are not sufficiently sensitive to permit reliable evaluation of the complex biological processes in the hepatocyte (biosynthesis, biotransformation, catabolism of xenobiotics, etc.) and of the changes in these processes in the presence of disease.
In addition, they are subject to a large number of external influences and are distorted by these. For example, they are to some extent distorted by the required therapeutic intervention, by replacement of human plasma, clotting factors or albumin, and can thus not be used as liver function parameters. Many different liver function tests have been described in the literature (Matsumoto, K., M. Suehiro, et al. (1987): “[13C]methacetin breath test for evaluation of liver damage.” Dig Dis Sci 32 (4): 344-8, 1987; Brockmoller, J. and I. Roots (1994): “Assessment of liver metabolic function. Clinical implications.” Clin Pharmacokinet 27 (3): 216-48).
However, it has not hitherto been possible, with any test method, to make valid and genuinely quantitative statements on liver function. In all methods to date, it was possible only to make a significant differentiation between different disease groups with already clinically detectable signs. Consequently, in clinical practice, no liver function test is employed in routine diagnostics, since these tests do not afford any additional clinical benefit based on their present accuracy.
The 13C-methacetin breath test used hitherto, with an exclusively oral administration of the substance, is a method which can distinguish between the liver function capacity of healthy subjects and that of patients with chronic hepatitis without cirrhosis and with cirrhosis in the different Child-Pugh stages (Matsumoto, K., M. Suehiro, et al. (1987): “[13C]methacetin breath test for evaluation of liver damage.” Dig Dis Sci 32 (4): 344-8, 1987), but does not permit a genuine quantification.
The substance methacetin is demethylated to paracetamol in a rapid one-step reaction by the enzyme CYP1A2 in the liver, with CO2 subsequently being produced. By 13C-labeling of the methyl group bonded via the ether bridge, 13CO2 can then be measured in the exhaled air. The following formula (I) represents the structural formula of methacetin:

The aim of genuine quantification with an individual measurement result cannot be achieved using the previous methods. There are two reasons for this:    1. The basis for statements derived from a breath test is that the step to be evaluated in the cascade of processes of absorption and metabolism has to be the step that determines the reaction rate. In the previous methods for evaluating the liver function (oral administration of the test substance), however, the rate-determining step is in most cases the absorption, not the conversion of the substrate in the liver.    2. To be able to make quantitative statements on the basis of an enzyme system (in the present case: to be able to determine the maximum liver function capacity, that is to say the functional liver capacity), the enzyme system to be tested has to be fully utilized at least in the short term. Only in this case does the reaction proceed independently of the substrate concentration.
For a genuine quantification, therefore, it is imperative to reach substrate surplus. If this is not achieved, the reaction rate is directly proportional to and therefore dependent on the substrate concentration, which for its parts drops non-linearly. A quantitative statement on functional capacity is impossible. In all studies using oral test substances, no genuine quantification could therefore take place, because full enzyme utilization is not achieved with the previous methods. This has the following causes:    1. When used orally, methacetin must first pass through the stomach and be transported as far as the duodenum and the proximal jejunum in order to be absorbed. Only then can the substance reach the liver by way of the portal vein. In principle, this process costs time and results in delayed and incomplete inundation in the liver. This is extremely variable and is influenced by numerous physiological and pathological conditions. For example, in cirrhosis of the liver, in which liver function tests could be used for staging and for therapy management, the intestinal transit and absorption is greatly changed (Castilla-Cortazar, I., J. Prieto, et al. (1997): “Impaired intestinal sugar transport in cirrhotic rats: correction by low doses of insulin-like growth factor I”. Gastroenterology 113 (4): 1180-7). In the period following abdominal operations too (e.g. liver resections or liver transplants), intestinal atony (paralytic ileus) means that no reliable statement can be made at all.    2. A sufficient dose of the test substance is necessary. With too low a dose, as in most methods for carrying out the oral methacetin breath test, full utilization of the enzyme system per se is not achieved.
It should also be noted that methacetin is extremely sparingly soluble in water or in an aqueous buffer. It crystallizes out of a usually aqueous solution within a period of hours to days. Such a solution can be used only, if indeed at all, for oral administrations of methacetin. Other administration forms are not possible.
Moreover, in the previous methods, the percentage recovery rate of the applied dose (dose %/h) and the cumulative dose are analyzed at specific times or time intervals in order to determine the liver function. The calculation of the dose %/h does not absolutely define the reacted substrate quantities and also does not take account of the individual bodyweight of the patient. It is not possible in this way to individualize and thus standardize the results in order to class the maximum functional liver capacity into a standard population.
The previous determination of the metabolized cumulative dose Dkum over a defined period of time is equally inexpressive in respect of functional liver capacity. For a reliable statement concerning the maximum conversion of the enzyme system over time, said system would have to be fully utilized over the entire period. For the reasons mentioned above, this is not the case. Consequently, the presently used calculation of the cumulative dose cannot be used for quantifying the functional liver capacity.
To transfer the air exhaled by an individual into a measurement device, it is recommended to use a respiratory mask which is placed onto the face of the individual. For the subsequent reliable conduct of an analysis method, it is critically important that the exhaled air is safely separated from the inhaled air and, in addition, that unforced breathing by the individual is permitted by a low airway resistance of the respiratory mask.
Various types of respiratory masks are in common use in medicine, in occupational safety and also in diving. In medicine, this is the case in the induction and performance of anesthesia or also for respiratory therapy and noninvasive ventilation. Masks with a good matching shape and a tight fit are preferred, and the required valves are fitted outside the masks, in the tube systems or in the other connected appliances.
Valves are installed in some masks used in occupational safety and also in masks used in diving, but the focus here lies in the delivery of respiratory gas and in the secure sealing of the system. High airway resistance generally arises in these cases, with the result that, for example, a medical test is needed to ensure suitability before occupational use of such a system.
To analyze certain constituents in the exhaled air, it is necessary to separate the respiratory gas path as close as possible to the site of origin of the exhaled substances, i.e. as close as possible to the pulmonary alveoli. Otherwise, the inhaled air and the exhaled air mix together. Moreover, the separation must not cause any substantial increase in airway resistance, especially not in the case of patients whose pulmonary function is compromised for whatever reason. The inhalation resistance specifically should not substantially increase, since the respiratory work or the supply of gas cannot be mechanically assisted as it is, for example, in anesthesia, ventilation, or in diving equipment or occupational safety equipment. Moreover, it is of great importance to establish, during the analysis, whether the respiratory mask is sitting tightly on the face or has possibly just been taken off.